The formation of filaments or whiskers, particularly from the surface of a pure metal, is a known phenomenon which can have consequences in the production of short-circuits or the initiation of electrical arcs at the surface of the metallic layers used for the plating of contacts of electronic components. The pure metal may be for example, tin, zinc, cadmium, indium, antimony, silver or gold, as opposed to an alloy composed of at least one metal and another element.
Certain factors such as mechanical or chemical stresses, scratches, diffusion of different materials, or thermal expansion increase the risk of filament formation. Contrary to dendrites, another phenomenon appearing at the surface of certain metals, the dissolution of the metal or the presence of electromagnetic fields does not have an effect on the formation or the growth of the filaments.
The contacts of electronic components are generally composed of a base substrate such as copper, plated with a metallic coating, of which pure tin (as opposed to a tin alloy) is most frequently used. This is particularly common for contacts, especially for press-fit contacts. When the contacts made of copper plated with pure tin, in addition to the stresses inherent in the coating layer or in the substrate itself, compression stresses in the plating often lead to the formation of filaments due to the diffusion of copper atoms from the substrate along the boundaries of the grains of the tin film. The filaments are often formed of an intermetallic compound (“IMC”), Cu6Sn5. Under such stresses, the tin filaments can be formed from grains, the orientation of which differs from the major orientation of the tin film or from the orientation of the adjacent grains.
Conventionally, tin filament formation is reduced through the use of platings made of alloys of tin and lead. However, since lead is currently banned for the majority of electronic applications, particularly in the European Union, incidents due to the formation of filaments in the electronic components have again become common. Therefore, there is a need to find novel solutions for the prevention of the formation of the filaments, particularly on the contacts of electronic elements.
One approach to the problem of the tin filaments formed on elements of press-fit contacts is disclosed in European Patent Application No. EP 2 195 885 B1, which discloses a process for manufacture of an electrical contact element using a layer forming a diffusion barrier applied on a base material, on which is then applied a plurality of a combination of at least two metallic layers formed of different metals and at least one of which is made of tin. The layer forming the diffusion barrier prevents mixture between the base material and the layers of the combination of metallic layers, which are then subjected to a thermal treatment, the temperature of which must not exceed the melting temperature of the tin, i.e. 232° C. By controlling the temperature, the elements of the combination of metallic layers mix by diffusion, with the outer layer being an alloy comprising at least two different metals.
Another approach is detailed in WO 2006/134665 A1, which discloses the use of a nickel sub-layer between the copper-based substrate and the tin plating film. The nickel sub-layer reduces the stresses applied to the layer of tin by the intermetallic compound Cu6Sn5 by creating a denser intermetallic compound Ni3Sn4. A thermal treatment not exceeding 0.65 to 0.8 times the melting temperature of the grains can be applied in order to reduce the internal stresses of the coating.
These approaches offer somewhat of a reduction in the formation of the tin filaments, the approaches are difficult to implement and the production of coatings is not financially viable due to the duration and cost of the processes used. Due to the stresses and therefore to the permanent stresses applied to the ends of the press-fit contact pins by the printed circuit boards and deformations and displacements of the coating itself, these solutions are insufficient to avoid the formation of filaments in the case of press-fit contacts.